I would have set the coast diff higher and the power diff lower if I had realised you could
You can't, you can only adjust diff pre-load, I guess that's what Pete is referring too.
I've just had a go in it, and I agree it's a tricky little bugger.. I think some of that entry oversteer is down to the brake balance being set a little too far rearward. A couple of clicks forward helped to bring a little stability, at least for me. The power delivery is also a little peaky, and can lead to bit of unpredictability when it comes on cam. Changing the gear ratios to bring them a little closer together made it a lot more drivable (again, at least for me - setups can be very personal things..!). Once I had made those changes I actually found that reducing the diff pre-load (and so inducing a bit more lift-off oversteer) was then beneficial. A few other minor tweeks and for me it was faster and more stable. Couldn't match my Mustang times, but it would be competitive on tracks with more corners and less straights.